It is known that ethynylbenzenes may be produced by means of a sequence of halogenation and dehydrohalogenation reactions. For example, phenylacetylene may be prepared by the addition of bromine to styrene to produce .alpha.,.beta.-dibromoethyl benzene followed by the removal of two molecules of hydrogen bromide from the dibromo compound by treatment with an alkali. A modification of this procedure involves the bromination of ethylbenzene. For example, the bromination of diethylbenzene followed by dehydrobromination with sodium hydroxide produced diethynylbenzene monomers. This process suffers from the disadvantage that more than twice as much bromine is required as in the present invention and half of that excess evolves as HBr that has to be recovered in addition to the bromide generated in the dehydrobromination step.
In U.S. Pat. No. 4,120,909 while ethynylbenzenes were prepared from methyl phenyl ketones, it was stated that a similar procedure could be used to prepare meta or para diethynylbenzene from the corresponding divinylbenzene or diethylbenzene as the starting material. However, these halogenation-dehydrohalogenation processes have been described as expensive, complicated and giving comparatively low yields of the desired products.
Some processes used chloroform as the solvent which, as a suspected carcinogen, is now all but banned from use in commercial processes. For example, A. S. Hay, in Volume 25 of the Journal of Organic Chemistry at page 637 (1960), disclosed a process for brominating commercial divinylbenzene in chloroform with the separation of its various components via molecular distillation. Molecular distillation is not a practical procedure for commercial applications and the use of chloroform presents environmental problems in the workplace.
N. N. Lebedev, et al. in 84 Chem. Abstracts 105109e (1976) discloses the use of "technical" divinylbenzene (52% of a mixture of meta and para divinylbenzene and 34% of a mixture of meta and para ethylstyrene) as a starting material. Chlorine in dimethyl formamide was used to halogenate the monomer, and potassium hydroxide in isopropanol was used to convert the chlorinated intermediate into diethynylbenzene, which was obtained in 45% yield. In the present invention, the use of a high purity grade of divinylbenzene (about 78% pure) is taught, with a much higher yield of diethynylbenzene.
More recently, Neenan et al. in Vol. 53, Journal of Organic Chemistry, 2489 (1988) discloses the reaction of 1,3-dibromobenzene with trimethylsilylacetylene (TMSA) using dichlorobis(benzonitrile)palladium, triphenylphosphine, and copper (I) iodide as catalyst. Their initial attempt to distill the diethynylbenzene by this method resulted in an exothermic reaction and an explosion. Therefore, it was stated that these compounds should be distilled at high vacuum and at temperatures of less than 60.degree. C. in well-shielded equipment. They note that the absence of an inexpensive route to produce the diethynylbenzene monomers has discouraged the full development of polyacetylenic aromatic compounds.
Polyarylacetylene resins have been the subject of extensive research leading to numerous patents because of their excellent properties. They are capable of being molded with little or no evolution of volatiles and can be cured simply by heating. They have good electrical properties and are stable to high temperatures. Furthermore, because of the high carbon yields produced upon their pyrolysis (85-90%) they are excellent precursors for carbon-carbon composites in ablative applications. For these reasons and others as well, resins of this type have been the object of investigation for many years. Attempts have been made to develop these resins as commercial materials, mainly by the General Electric Company and by Hercules Incorporated. However, their full commercial development has been discouraged by the absence of an inexpensive, safe route to the diethynylbenzene monomers as noted in the 1988 article by Neenan.
Now for the first time, an improved, inexpensive synthesis process for preparation of such diethynylbenzene monomers is provided. This process provides for the preparation of the thermally sensitive monomers in a one pot reaction using readily available materials at low temperatures in an environment capable of absorbing large amounts of energy. It further provides a process that produces a high yield of product with a significantly reduced requirement for bromine in the bromination phase of the process. In the present invention, divinylbenzene is first brominated and then dehydrobrominated, followed by distillation to recover the diethynylbenzene product. More specifically, a brominated divinylbenzene product is produced by continuously and simultaneously combining mixed isomers of divinylbenzene and bromine in a sulfolane solution wherein said divinylbenzene is present in a concentration not exceeding 5 percent by weight, wherein the temperature is between 0.degree.-50.degree. C., and wherein sufficient bromine is added to the reaction mixture to react with all olefinic unsaturation of the divinylbenzene. The quantity of bromine is not to be more than 10 percent more than an amount necessary to react with all of said olefinic unsaturations and the total amount of divinylbenzene added to the reaction mixture is between 20-50 percent by weight of the sulfolane solvent. The next step is to react the treated brominated divinylbenzene product with a dehydrobromination caustic agent such as sodium hydroxide or potassium hydroxide. An excess of the agent is added to total about 50-250 mole % more than the amount required to remove all organically bound bromine from the brominated divinylbenzene product as a corresponding halide salt. The caustic agent is added at a rate that maintains the temperature of the solution in the range of from about 20.degree.-50.degree. C. Addition of the caustic agent is continued until the excess of the agent is present and then the reaction with the caustic agent is completed by heating the dehydrobrominated product to a temperature between about 90.degree. to about 100.degree. C. for a period of about 1/2 to 4 hours. The excess dehydrobromination caustic agent and the halide salts are then removed and a layer is isolated comprising the sulfolane and diethynylbenzene. The final step in the process is to recover the diethynylbenzene from the isolated layer.
In an alternative procedure, all of the divinylbenzene is placed in the reaction vessel with the sulfolane solvent followed by gradual addition of bromine.